Shredding and defiberizing machine

ABSTRACT

A machine for shredding and defiberizing papermaking material and extracting refined pulp therefrom, the machine including a substantially dome-shaped rotor having outwardly extending blades moving over a cooperating stationary bedplate and method of construction thereof, the blades being organized into groups having varying heights so as to achieve optimum circulation or horsepower utilization or both as a function of consistency.

United States Patent [191.

Honeyman 1 Oct. 22, 1974 1 SHREDDlNG AND DEFIBERIZING MACHllNE [75]Inventor: Robert Blakeley Honeyman, Carmel,

Calif.

[73] Assignee: Morden Machines Company,

Portland, Oreg. [22] Filed: Feb. 6, 1973 [21] Appl. No.: 330,081

3,339,851 9/1967 Felton et a1 241/4617 3,486,702 12/ l 969 Kmeco3,713,594 l/1973 Biakley ct al. 241/4617 Primary ExaminerGranville Y.Custer, Jr.

Attorney, Agent, or Firm-Dawson, Tilton, Fallon &

Lungmus [5 4 ABSTRACT A machine for shredding and defiberizingpapermaking material and extracting refined pulp therefrom, the

machine including a substantially dome-shaped rotor. having outwardlyextending blades moving over a cooperating stationary bedplate andmethod of construction thereof, the blades being organized into groupshaving varying heights so as to achieve optimum circulation orhorsepower utilization or both as a function of consistency.

9 Claims, 12 Drawing Figures 520m wmmo: 33mm 0% com 0mm 08 m? w {I WdUPAIENTEBBM 22 m4 Omw wwm

was

I SHREDDING AND DEFIBERIZING MACHINE BACKGROUND AND SUMMARY OF THEINVENTION For example, a paper manufacturer might operate the pulper fora continuous machine with a pulp con sisting of 3-4 percent. However, onother occassions (as for batch operations or for other considerations)the consistency may be 6 percent or higher. Also, the capacity (cubiccontents of the tank) often had determined the design, particularly ofthe rotor so that for different sized installations, the easy way hadbeen to extrapolate as contrasted to tailoring the machine for theintended usage. Still further, even when optimal values of powerutilization for a given consistency and capacity are approached, thereis lacking the cireulation necessary to achieve efficient pulping.

The inefficiencies (and inequities when viewed from the standpoint of anecology-ninded public) are avoided through the practice of the inventionwhere a procedure and structure are provided utilizing varying heightblades according to power, consistency, capacity and circulationrequirements.

Further, a specific preferred form of machine is provided wherein therotor assembly, which moves over the face of the stationary cooperatingbedplate, has a plurality of blades extending the same radial distancefrom the rotor hub. The bottom faces of the blades have the same shapeand surface area, decreasing in width from the hub to the blade tip. Thetop surfaces of the blades are flat and are of identical width, but,although they leave the hub at approximately the same radial distancefrom the axis of rotation of the rotor, they do not all have the sameslope with respect to the bedplate and consequently the blades vary inheight being arranged in more or less symmetrical order around the hub,the height and number of the corresponding blades depending on thegeneral nature of the material being treated and the operatingrequirements. I have found it advantageous to provide from about 2 toabout 4 times as many lower height blades as compared with greaterheight blades. The leading face of each blade is substantiallyperpendicular to the plane of the bedplate and extends obliquely awayfrom the direction of rotation of the rotor. The trailing face of eachblade has a concave downward slope from the rear edge of the top face.The bottom face of the blades are identically grooved to form barsparallel to the leading face of the blade and the grooves and barsextend in under the periphery of the hub preventing the possibility ofany material on the bedplate escaping treatment by getting in under thehub. This feature also will prevent extraneous trash material fromcollecting inwardly beneath the rotor hub. The stationary bedplate forbatch operation usually will have radiallyextending bars. For continuousoperation the bedplate will have suitable perforations, or radial barswith perforations in the grooves, all of which are well-known.

In a modification of the machine for batch operations the bedplate isperforated and extends over an annular recirculation chamber which hasinwardly-located discharge ports leading upwardly to the underside ofthe rotor hub with the result that a portion of the semidefiberizedmaterial which is forced down through the perforations in thebedplatewill be drawn up into the low pressure zone beneath the hub andsubjected to further defiberizing action without again traveling thecomplete path of the material being circulated throughout the tank.

DESCRIPTION OF THE DRAWINGS FIGS. 3, 4 and 5 are an end elevation andcrossseetional elevations of one of the higher blades on the rotor,taken on lines 3-3, 4-4 and 55, respectively of FIG. 1 and drawn to aconsiderably enlarged scale;

FIG-6 is a fragmentary bottom view of the rotor taken on the lineindicated at 6-6 in FIG. 2, but drawn to the same scale as FIG. 1;

FIG. 7 is a fragmentary top plan view of a modified form of rotor, drawnto the same scale as FIG. 1, but showing a portion of a typicalcooperating stationary bedplate employing small perforations;

FIG. 8 is sectional elevation of the rotor with the bedplate of FIG. 7taken along the axis of the rotor and thus along the line indicated at8-8 in'FIG. 7, drawn to the same scale as FIG. 2, and showing also thedischarging chamber beneath the perforated bedplate when the machine ismounted in the tank for continuous operation as opposed to batchoperation;

FIG. 9 is a sectional elevation of the rotor and bedplate showing themachine installed in the tank for a modified operation embodyingrecirculation of portions of the semi-treated material for batchoperations;

FIG. 10 is a top plan view of the bedplate used in the modifiedoperation illustrated in FIG. 9 and taken on line 10-10 of FIG. 9;

FIG. 11 is a chart of performance characteristics (speed versus power)of different kinds of rotors; and

FIG. 12 is a chart similar to FIG. 11 but relating speed to circulation.

Inasmuch as the invention in its broader aspects can be betterappreciated by relating it to specific mechanical embodiment, referenceis first made to FIGS. 1-6. The machine shown there includes a rotor,indicated in general by R, which operates in conjunction with acooperating, stationary bedplate. The rotor has a hollow, round,substantially dome-shaped hub 10 which is sewhich rests on theperipheral portion of the mounting member flange 11 and is firmlysecured thereto by a series of recessed screws 14. The hub may beadjusted with respect to the mounting member when desired by theinterposition of shims 15 between the hub 10 and the flange 11' of themounting member for a reason later apparent.

The hub 10 of the rotor R is formed with plurality of integral blades 16starting on the hub at approximately the same radial distance from theaxis of rotor rotation and extending the same distance beyond theperiphery of the rotor hub. The bottom faces 16 of these blades allextend in the same plane perpendicular to the axis of rotation and in aplane parallel to the plane of the stationary bedplate later mentioned.While the blades 16 are of various heights, as presently explained, thebottom faces 16' are all identical in size and shape (see FIG. 6) anddecrease in width from the hub to their outer tips to form a web behindthe leading face of each blade. It will be appreciated however, that thebottom faces 16' and the defibering surface of the bedplate may be aflat surface or a surface of revolution as the frustro-conical surfacedescribed in US. Pat. No. 2,858,990. However, it has been found moreadvantageous to provide the flat surface herein defined.

As shown by FIGS. 1 and 6 in which the arrows X indicate the directionof rotation of the rotor, the leading face 17 of each blade extendsobliquely away from the direction of rotation of the rotor. Also, theleading face of each blade is substantially perpendicular to the commonplane of the bedplate. The bottom faces 16' of the rotor blades extendin under the periphery of the rotor hub designated 18 in FIGS. 2 and 6.It is understood that the rotor can be manufactured for rotation in theopposite direction of that shown by reversing the design of rotorblades.

The bottom face 16' of each of the blades is formed preferably withequally spaced bars 19 (FIG. 6) of identical width, extending parallelto the leading face of the blade. In theillustration given, the barsextend from the outer end of the blade inwardly under the hub. Asillustrated, these may have a common inner termination. The bottom faceof each blade has four such bars, the bars being of varying length. Theinner end of the last and shortest bar on the bottom face of each bladeterminates close to the inner end of the first and longest bar on thebottom of the next succeeding blade. The stationary bedplate 20, in themachine of FIGS. 1 and 2, extends in a plane perpendicular to the axisof rotation of the rotor and has central opening to accommodate theupper portion of the rotor shaft 12 with a suitable sealing ring aroundthe shaft. This bedplate is secured by screws to a base plate 21 ofsimilar size, which in turn is secured in a corresponding recess in thewall of the tank 23 in which the treatment of the pulp material takesplace. The peripheral portion of the bedplate 20, starting from insidethe hub periphery, is grooved so as to form a series of bars 22extending radially out to the periphery of the bedplate and thuscooperating with the bars on the bottom faces of the rotor blades toproduce the desired defiberizing action on the material passing betweenthe opposed sets of bars. The amount of clearance between the opposedsets of bars can be adjusted as desired, by means of the interposedshims 15 (FIG. 2) between the rotor hub 10 and the flange 11' of therotor mounting member 11 previously mentioned. It is appreciated thatbedplate bars may assume other patterns than radial.

The fact that the bars 19 on the bottom faces 16 of the blades 16 extendin under the periphery of the rotor hub is an advantageous feature sincethis renders less likely the collecting and building up of damagingextraneous material, between the rotor and the bedplate, in the areawithin the rotor hub periphery. The bottom face of the bottom flange 11'of the mounting member for the rotor hub also is preferably formed withobliquely-extending bars 11a (FIG. 6) to engage and throw out anymaterial which might possibly enter into this space.

Although the bottom faces of the rotor blades are exactly the same sizeand shape, the blades are not all the same height. As in US. Pat. No.2,858,990, the higher blades act to facilitate and hasten the breakingup of large pieces or slabs of material encountered by the retor.However, too large a number of higher blades would negate thedestructive tearing effect of the proportionately fewer blades, on largemasses of material and, also, result in uneconomical power consumption.Further, too many blades in total will tend to obstruct the flow ofpartially treated material into the defiberingzone at the surface of thebedplate and also can restrict materially the pumping or circulatingcapacity of the rotor. In the particular example illustrated, the rotoris shown with a total of twelve blades which are of three differentheights, as indicated somewhat diagrammatically in FIG. 2, the highestblades being indicated by the reference a, intermediate blades by thereference b, and the low blades by the reference c. When the rotor ismade with twelve blades, it is possible to provide four blades of thethree heights a, b, and c symmetrically arranged, or, depending upon thenature of the material being treated, it is possible, as anotherexample, to have three of the high blades 0, with each followed by twoof the intermediate blades b, and one low blade c symmetricallyarranged. The selection of the proper arrangement and height of theblades with respect to the particular nature of the material beingtreated and the proper rotor speed make possible the most efficient andeconomical power consumption.

As previously mentioned, the leading face of each rotor blade ispreferably flat and substantially perpendicular to the plane of thebedplate, extending out obliquely in a direction away from the directionof rotation. The top face of each blade is flat and narrow with paralleledges, the slope of the top face with respect to the plane of thebedplate depending upon the height of the blade at its tip. The trailingface of each blade slopes downwardly toward the succeeding blade, whichdownward slope becomes more and more concave inwardly along the blade,and this concave trailing face aids in feeding the material down to thebedplate and to the leading face of the succeeding blade. FIGS. 3, 4 and5 indicate an end elevation, an intermediate crosssection, and across-section taken further inwardly respectively of one of the highblades of the rotor.

In FIGS. 7 and 8 of the rotor R is shown in combination with a bedplate24 which, instead of having radial bars to cooperate with the bars onthe bottom faces of the rotor blades, is perforated in the annular areaover which the rotor blades pass, and extending beyond this area whenadditional capacity is required. An annular discharging passageway 25(FIG. 8), provided in the tank housing beneath the annular perforatedarea of the bedplate, leads down into an annular chamber 26 whichpreferably has a sloping wall leading to a controlled outlet pipe 27.The use of perforated bedplates for continuous operation in machines fortreating paper stock is in itself well-known and need not be furtherdescribed. However, the rotor R of the present invention with the novelblades, when used in combination with such a perforated bedplate, alsomakes possible more efficient and economical performance in cases Wherecontinuous operation is desired as opposed to batch operations.

FIGS. 9 and 10 illustrate a modification of the machine for batchoperations in which semi-defiberized material receives additionaldefiberizing action without having again to travel the entirecirculation pattern of the material in the tank surrounding the rotor.In FIGS. 9 and 10 of the bedplate 28 is formed with perforations, orwith radial bars 29 with perforations 30 in the grooves between thebars, (in the annular path over which the rotor blades pass). An outerannular chamber 31 beneath the annular perforated portion of thebedplate is connected with an inner annular chamber 32. The bedplate 28is also provided with relatively large openings 33 above the innerchamber 32, which openings lead upward to the under side of the mountingmember flange 11' of the rotor. The material being treated is forceddownwardly through the perforated bedplate by the pressure wavespreceding the advancing rotor blades and into chamber 31 thence flowinginto chamber 32 and up into the low pressure zone beneath the rotorthrough ports 33. There it re-enters the defibering zone and is carriedoutwardly again by centrifugal action of the rotor blades, being subjectin this way to additional defiberizing action beneath the rotor blades.

Reference is now made to FIG. 11. As indicated, this is a graph or chartof performance characteristics of certain rotors. More specifically, thenumeral 363 designates the plot relating speed in rpm as a function ofbrake horsepower for a rotor having 12 blades of differ ent sizes,provided in three groups. This is illustrated in part in FIG. 7 where itwill be noted that there are three high blades 0, six intermediateheight blades b and three low blades c. In FIG. 12, the performancecharacteristic of the surface circulation rate in fpm is related to thespeed of the rotor and the plot relating these for the embodiment of theinvention seen in FIG. 7 is also designaged 363.

In FIGS. 11 and 12, there is seen two other plots relating performancecharacteristics. These plots are designated 306 and 309 and relate,respectively, to a 9 blade rotor and a 12 blade rotor, the 9 bladerotor, for example having three high blades a and six low blades 0,arranged with two low blades following each high blade. The 12 bladerotor has 3 low blades following each high blade. In some instances the9 blade rotor may be developed from the same pattern as the 12 bladerotor but eliminating one of the low blades after each high blade. Insuch a case the blade spacing is not equal. However the rotor is stillbalanced.

Still further, in FIGS. 11 and 12, the plots 114 relate to theperformance characteristics of the commercial embodiment of US. Pat. No.2,858,990, the series FGVl 14 machine of the assignee Modern MachinesCompany of Portland, Oregon of the aforesaid patent.

In the Series FGV-l 14 machine, there are three high blades a andtwenty-one low blades 0 arranged with seven low blades c following eachhigh blade a. The data for the plots of FIGS. 11 and 12 were obtainedwith the other variables, viz., size or capacity and stock consistencybeing the same.

Referring now to FIG. 12, it will be noted that to achieve a circulationrate of fpm in the Series FGV- I 14 machine (see point 34 on plot 114),it is necessary to employ in excess of 250 brake horsepower thatcorresponding to the point 35 on plot 114 in FIG. 11. On the other hand,to achieve the same circulation rate utilizing the blade arrangement ofFIG. 7, see point 36 in FIG. 12, it is necessary to use only about I25brake horsepower as determined by the abscissa corresponding to thepoint 37 in FIG. 11. In like fashion, the power requirementsaresubstantially less for the rotor configurations responsible for theperformance characteristics 306 and 309 when compared with the SeriesFGV-l 14 machine rotor responsible for the plots 114-.

Alternatively, it will be appreciated that the invention provides anadvantageous method by which circulation rates can be increased withoutincreasing power.

The plot in FIGS. 11 and 12 are not single line curves but rather areranges or bands based upon a number of tests. The speed and powerfigures are correlated to a 27 inch diameter rotor as extrapolated fromlaboratory models this being considered a proven approach in this field.The tests were correlated to a given size commercial pulping unitinstalled in a maximum size tank. All tests treated stock of identicalspecifications at 6 percent A.D. consistency. Data was obtained using adynomatic variable speed drive with power corrected by formula to actualbrake horsepower output and correlated to commercial experience. Thedata is typical of an extensive program numbering over tests. In themaximum size tank (in proportion to rotor diameter) as used in theseries of tests referred to, it was determined that the allowableminimum surface velocity of the stock at a given location in the tankwould be 50 fpm to provide adequate mixing and submergence andtherefore, is used as a point of comparison.

The plots were bracketed to show the range or band that the respectiverotors fall into. The circulation curves will showa greater range,primarily due to air entrainment at higher speeds, and also to minoruncontrollable variables in stock viscosity. In the comparisons shown,the averages or mid-points of the range of curves was selected.

The 306 rotor is designed for medium or large tanks at low consistency,or small tanks at high consistency. The 309 rotor is for generalpurpose, or intermediate applications. The 363 rotor is for highconsistency, and, or large tank applications. Atequal or betterdefibering efficiencies, as rated by horsepower per day, per ton ofstock, the new series rotors show clear advantages over the FGV-1l4series.

Through the employment of a number of high blades in combination withlow and/or intermediate height blades, I am able to obtain improvedcirculation rates without the requirement of additional power orsubstantial savings in power without sacrifice or optimum rates ofcirculation. For this purpose, it is advantageous to provide the sum ofthe intermediate and lowheight blades to be about twice or more thenumber of the high blades a. Optimally, the ratio of interediate and lowheight blades to the high blades is between 2 and I the b blades 2 /2inches and the blades 1 /z inches.

The heights, size and configurations of the rotor blades, in combinationwith any of the bedplates mentioned, provide several advantages. Theyenable the material or slurry in the tank to be circulated at animproved level of efficiency relative to power usage. The high bladesperform the necessary, primary breakdown of the larger pieces or slabsof material enhanced by the leading faces of all the blades providing anabrupt oblique angle of attack (relative to rotation) on the materialencountered. The hazard of having any tramp metal or other foreignmaterial reaching the defibering zone is practically eliminated whenincorporating rotor blades that are substantially perpendicular to afiat bedplate. By substantially perpendicular, I include faces thatslope slightly rearwardly but not significantly forwardly; The trailingface of each blade acts to feed the material into the space ahead of thesucceeding blade, but the blade contours are also so designed as toprevent any blinding or plugging of the rotor by the materialencountered, and the bottom faces of the blades provide proportionatelylarge, working or defiberizing surfaces acting in conjunction with anyof the stationary bedplates illustrated to accomplish the desireddefiberizing results efficiently and with an economical consumption ofpower.

I claim:

1. A machine for shredding and defiberizingpapermaking materialincluding a rotor, means for rotating said rotor about an axis, astationary bedplate formed into a surface of revolution about said axisof rotation and defining a plurality of shear edges providing agenerally planar defibering zone, a substantially domeshaped hub on saidrotor, a plurality of blades extending from said hub, each blade havinga predetermined height and including a substantially flat forward worksurface extending perpendicular to said bedplate and forming an obliqueangle with a radius of said rotor away from the directionof rotation,the trailing surface of each blade further defining a concave downwardslope along a portion thereof from the rear edge of top face for drawingmaterial downinto an intermediate location on the working face of asucceeding blade, the bottom faces of said blades extending in a surfaceparallel to and spaced slightly from said bedplate, and defining aplurality of bars each providing a straight shearing edge, said bladesincluding a first set of blades of a first predetermined height spacedsymmetrically about said axis of rotation and a second set of blades ofa second predetermined height less than said first height and spacedsymmetrically about said axis, the ratio of the number of blades in saidsecond set to the number of blades in said first set being in the rangeof 2-4 to 1.

.2. The structure of claim 1 wherein each blade includes a trailing webportion for carrying said bars, the concavity of said trailing surfaceof said blades increasing in a direction inwardly along each blade, thecurvature of the trailing surface of each blade extending out along itsassociated web portion.

3. The structure of claim 2 wherein said bars beneath said blades areparallel to said work surface and extend inwardly beneath the peripheryof said hub to force material from beneath said hub outwardly undercentrifugal force.

4. The structure of claim 3 wherein said second set of blades includesblades of at least two different heights other than the height of saidfirst set of blades.

5. A machine for shredding and defiberizing papermaking materialincluding a rotor, means for rotating said rotor about an axis, astationary bedplate formed into surface of revolution about said axis ofrotation and defining a plurality of shear edges providing a generallyplanar defigering zone, a substantially domeshaped hub on said rotor, aplurality of blades extending from said hub, each bladehaving apredetermined height and including a substantially flat forward worksurface extending perpendicular to said bedplate and forming an obliqueangle with a radius of said rotor away from the direction of rotation,the trailing surface of each blade further defining a concave downwardslope along a portion thereof from the rear edge of the top for drawingmaterial down into an intermediate location on the working face of asucceeding blade, the bottom faces of said blades extending in a surfaceparallel to and spaced slightly from said bedplate and defining aplurality of bars each providing a straight shearing edge, said bladesincluding a first set of blades of a first predetermined height spacedsymmetrically about said axis of rotation, a second set of blades of asecond predetermined height less than said first height and spacedsymmetrically about said axis, one on either side of an associated bladeof said first set, and a third set of blades of a third predeterminedheight spaced symmetrically about said axis of rotation and equidistantbetween said blades of said first set, the number of blades in saidfirst and third sets being equal, and the number of blades in saidsecond set being twice the number of blades in said first set.

6. The structure of claim 5 wherein the relationship of the height ofeach of said blades of said first set, said second set and said thirdset is approximately respectively 3.5 to 2.5 to 1.5.

7. The structure of claim 5 wherein each blade includes a trailing webportion for carrying said bars, the concavity of said trailing surfacesof said blades increasing in a direction inwardly along each blade andextending out along an associated web portion.

8. The structure of claim 7 wherein the bars beneath said blades areparallel to said work surface and extend inwardly under the periphery ofsaid hub to force material from beneath said hub outwardly undercentrifugal force.

9. A machine for shredding and defiberizing papermaking materialincluding a rotor, means for rotating said rotor about an axis, astationary bedplate extending in a plane generally perpendicular to saidaxis of rotation, a substantially dome-shaped hub on said rotor, a firstplurality of equally spaced blades having their top edges starting fromsaid hub at approximately raannular chamber inwardly of a the defiberingzone, whereby material to be treated passing through said bedplate fromsaid defibering zone into said first annular chamber is forced into saidsecond annular chamber and thence upwardly through said enlargedopenings to be recirculated through said defibering zone.

1. A machine for shredding and defiberizing paper-making materialincluding a rotor, means for rotating said rotor about an axis, astationary bedplate formed into a surface of revolution about said axisof rotation and defining a plurality of shear edges providing agenerally planar defibering zone, a substantiAlly dome-shaped hub onsaid rotor, a plurality of blades extending from said hub, each bladehaving a predetermined height and including a substantially flat forwardwork surface extending perpendicular to said bedplate and forming anoblique angle with a radius of said rotor away from the direction ofrotation, the trailing surface of each blade further defining a concavedownward slope along a portion thereof from the rear edge of top facefor drawing material down into an intermediate location on the workingface of a succeeding blade, the bottom faces of said blades extending ina surface parallel to and spaced slightly from said bedplate, anddefining a plurality of bars each providing a straight shearing edge,said blades including a first set of blades of a first predeterminedheight spaced symmetrically about said axis of rotation and a second setof blades of a second predetermined height less than said first heightand spaced symmetrically about said axis, the ratio of the number ofblades in said second set to the number of blades in said first setbeing in the range of 2-4 to
 1. 2. The structure of claim 1 wherein eachblade includes a trailing web portion for carrying said bars, theconcavity of said trailing surface of said blades increasing in adirection inwardly along each blade, the curvature of the trailingsurface of each blade extending out along its associated web portion. 3.The structure of claim 2 wherein said bars beneath said blades areparallel to said work surface and extend inwardly beneath the peripheryof said hub to force material from beneath said hub outwardly undercentrifugal force.
 4. The structure of claim 3 wherein said second setof blades includes blades of at least two different heights other thanthe height of said first set of blades.
 5. A machine for shredding anddefiberizing papermaking material including a rotor, means for rotatingsaid rotor about an axis, a stationary bedplate formed into surface ofrevolution about said axis of rotation and defining a plurality of shearedges providing a generally planar defigering zone, a substantiallydome-shaped hub on said rotor, a plurality of blades extending from saidhub, each blade having a predetermined height and including asubstantially flat forward work surface extending perpendicular to saidbedplate and forming an oblique angle with a radius of said rotor awayfrom the direction of rotation, the trailing surface of each bladefurther defining a concave downward slope along a portion thereof fromthe rear edge of the top for drawing material down into an intermediatelocation on the working face of a succeeding blade, the bottom faces ofsaid blades extending in a surface parallel to and spaced slightly fromsaid bedplate and defining a plurality of bars each providing a straightshearing edge, said blades including a first set of blades of a firstpredetermined height spaced symmetrically about said axis of rotation, asecond set of blades of a second predetermined height less than saidfirst height and spaced symmetrically about said axis, one on eitherside of an associated blade of said first set, and a third set of bladesof a third predetermined height spaced symmetrically about said axis ofrotation and equidistant between said blades of said first set, thenumber of blades in said first and third sets being equal, and thenumber of blades in said second set being twice the number of blades insaid first set.
 6. The structure of claim 5 wherein the relationship ofthe height of each of said blades of said first set, said second set andsaid third set is approximately respectively 3.5 to 2.5 to 1.5.
 7. Thestructure of claim 5 wherein each blade includes a trailing web portionfor carrying said bars, the concavity of said trailing surfaces of saidblades increasing in a direction inwardly along each blade and extendingout along an associated web portion.
 8. The structure of claim 7 whereinthe bars beneath said blades are paralLel to said work surface andextend inwardly under the periphery of said hub to force material frombeneath said hub outwardly under centrifugal force.
 9. A machine forshredding and defiberizing papermaking material including a rotor, meansfor rotating said rotor about an axis, a stationary bedplate extendingin a plane generally perpendicular to said axis of rotation, asubstantially dome-shaped hub on said rotor, a first plurality ofequally spaced blades having their top edges starting from said hub atapproximately radial distance from the axis of rotation of said rotor,the bottom faces of said blades extending in a plane parallel to andspaced slightly from said bedplate to define a defibering zone, saidbottom faces of said blades defining a plurality of bars providingshearing edges, a second plurality of blades extending from said hub andhaving bottom faces identical to the blades in the first set, and beingof lesser height, the ratio of the number of blades in said second setto the number of blades in said first set being in the range of about2-4 to 1, said stationary bedplate being perforated in an annular regionbeneath said defibering zone, a first annular chamber beneath saidbedplate and said defibering zone, a second annular chamber spacedinwardly from and in communication with said first annular chamber, saidbedplate having enlarged openings above said second annular chamberinwardly of the defibering zone, whereby material to be treated passingthrough said bedplate from said defibering zone into said first annularchamber is forced into said second annular chamber and thence upwardlythrough said enlarged openings to be recirculated through saiddefibering zone.